how to build fusion lasers

The first nuclear bombs relied on Uranium 235, which upon splitting radiates 2 neutrons, creating a chained reaction, albeit with the help of some other explosives or kinetic force to condense the radioactive element to critical mass.

There are other ways however. The 3rd nuclear core that might've been dropped on Japan had it not surrendered after the 2nd, was used in experiments that lead to the deaths of a number of curious scientists. By trapping the dubbed "Demon Core", a chunk of uranium, inside a material that reflects neutron radiation, the required mass for critical mass can be greatly reduced. Thus, by creating a sufficiently effective and heat-resistant neutron-deflecting material, one could, finally, achieve radiation lasers